The preparation of di- and polyamines of the diphenylmethane series (MDA) by reaction of aniline with formaldehyde in the presence of acidic catalysts is known in principle. In the context of the present invention, di- and polyamines of the diphenylmethane series are understood to mean amines and mixtures of amines of the following type:
n here is a natural number≥2. The compounds of this type in which n=2 are referred to hereinafter as diamines of the diphenylmethane series or diaminodiphenylmethanes (MMDA hereinafter; “monomer MDA”). Compounds of this type in which n>2 are referred to in the context of this invention as polyamines of the diphenylmethane series or polyphenylene polymethylene polyamines (PMDA hereinafter; “polymer MDA”). Mixtures of the two types are referred to as di- and polyamines of the diphenylmethane series (for the sake of simplicity referred to hereinafter as MDA). The corresponding isocyanates that can be derived in a formal sense by replacing all NH2 groups with NCO groups from the compounds of the formula (I) are accordingly referred to as diisocyanates of the diphenylmethane series (MMDI hereinafter), polyisocyanates of the diphenylmethane series or polyphenylene polymethylene polyisocyanates (PMDI hereinafter) or di- and polyisocyanates of the diphenylmethane series (MDI hereinafter). The higher homologs (n>2), both in the case of the amine and in the case of the isocyanate, are generally always present in a mixture with the dimers (n=2), and so only two product types are of relevance in practice, the pure dimers (MMDA/MMDI) and the mixture of dimers and higher homologs (MDA/MDI). The position of the amino groups on the two phenylene groups in the dimers (para-para; ortho-para and ortho-ortho) is specified hereinafter only when it is important. For the sake of simplicity, this is done in the X,Y′-MDA form (4,4′-, 2,4′- or 2,2′-MDA), as is customary in the literature. The same is true of MDI (identification of the isomers as X,Y′-MDI (4,4′-, 2,4′- or 2,2′-MDI).
Industrially, the di- and polyamine mixtures are converted predominantly by phosgenation to the corresponding di- and polyisocyanates of the diphenylmethane series.
The continuous or partly discontinuous preparation of MDA is disclosed, for example, in EP-A-1 616 890, U.S. Pat. No. 5,286,760, EP-A-0 451 442 and WO-A-99/40059. The acidic condensation of aromatic amines and formaldehyde to give di- and polyamines of the diphenylmethane series proceeds in multiple reaction steps. In what is called the “aminal process”, in the absence of an acidic catalyst, formaldehyde is first condensed with aniline to give what is called aminal, with elimination of water. This is followed by the acid-catalyzed rearrangement to MDA in a first step to give para- or ortho-aminobenzylaniline. The aminobenzylanilines are converted to MDA in a second step. Main products of the acid-catalyzed reaction of aniline and formaldehyde are the diamine 4,4′-MDA, its positional isomers 2,4′-MDA and 2,2′-MDA and the higher homologs (PMDA) of the various diamines. In what is called the “neutralization process”, aniline and formaldehyde are converted in the presence of an acidic catalyst directly to aminobenzylanilines, which are then rearranged further to give the bicyclic MMDA isomers and higher polycyclic PMDA homologs. The present invention relates to the aminal process.
Irrespective of the process variant for preparation of the acidic reaction mixture, the workup thereof, according to the prior art, is initiated by neutralization with a base. This neutralization is typically effected at temperatures of, for example, 90° C. to 100° C. without addition of further substances (cf. H. J. Twitchett, Chem. Soc. Rev. 3(2), 223 (1974)). It can alternatively be effected at a different temperature level in order, for example, to accelerate the degradation of troublesome by-products. Hydroxides of the alkali metal and alkaline earth metal elements are suitable as bases. Preference is given to using sodium hydroxide solution.
After the neutralization, the organic phase is separated from the aqueous phase in a separating vessel. The organic phase which comprises crude MDA and remains after removal of the aqueous phase is subjected to further workup steps, for example washing with water (base washing) in order to wash residual salts out of the crude MDA. Finally, the crude MDA thus purified is freed of excess aniline, water and other substances present in the mixture (e.g. solvents) by suitable methods, for example distillation, extraction or crystallization. The workup which is customary according to the prior art is disclosed, for example, in EP-A-1 652 835, page 3 line 58 to page 4 line 13, or EP-A-2 103 595, page 5 lines 21 to 37.
International patent application WO 2014/173856 A1 provides a process for preparing di- and polyamines of the diphenylmethane series by converting aniline and formaldehyde in the absence of an acidic catalyst to aminal and water, removing the aqueous phase and processing the organic aminal phase further to give the di- and polyamines of the diphenylmethane series, in which use of a coalescence aid in the phase separation of the process product obtained in the aminal reaction reduces the proportion of water and hence also of water-soluble impurities in the organic phase comprising the aminal. The di- and polyamines of the diphenylmethane series that are obtained by acid-catalyzed rearrangement and workup after further processing of the aminal phase are of excellent suitability for preparation of the corresponding isocyanates.
The quality of a reaction process for preparation of MDA is defined firstly by the content of unwanted secondary components and impurities in the crude product that can arise from improper conduct of the reaction. Secondly, the quality of a reaction process is defined in that the entire process can be operated without technical production outage or problems that necessitate intervention in the process, and that losses of feedstocks are prevented or at least minimized.
Although the prior art processes described succeed in preparing MDA with a high yield and without loss of quality in the end products, the only processes described are in the normal state of operation. Only a few publications are concerned with states outside normal operation:
International patent application WO 2015/197527 A1 relates to a process for preparing di- and polyamines of the diphenylmethane series (MDA), to a plant for preparation of MDA and to a method of operating a plant for preparation of MDA. The invention enables optimization of production shutdowns in the operation of the MDA process with regard to time taken and optionally also with regard to energy and material consumption by means of what is called a circulation mode in individual plant components. During an interruption in the process or interruption of the operation of individual plant components, there is no introduction of formaldehyde into the reaction, and the plant components that are not affected by an inspection, repair or cleaning measure are operated in what is called circulation mode. What this achieves, among other effects, is that only the plant component in question must be shut down for the period of the measure, which may be advantageous in terms of productivity and economic viability of the process and the quality of the products prepared.
International patent application WO 2015/197519 A1 relates to a process for preparing diamines and polyamines of the diphenylmethane series, in which care is taken during the running-down of the production process that an excess of aniline over formalin is ensured.
International patent application WO 2015/197520 A1 relates to a process for preparing diamines and polyamines of the diphenylmethane series (MDA) from aniline and formaldehyde, in which care is taken during the start-up procedure to ensure that there is a sufficient excess of aniline over formaldehyde which is at least 105% of the molar ratio of aniline to formaldehyde wanted for the target formulation of the MDA to be produced.
Changes in the target production capacity (also called “change in load”) during continuous production (i.e. with a starting state and an end state, in which MDA is produced) are not taken into account here either. Since aniline is typically used in stoichiometric excess, the production capacity of a given plant for preparation of MDA is defined by the formaldehyde feed which, in the context of this invention, is referred to as (formaldehyde) load.
Changes in load are recurrent plant states that have a considerable effect on the economic (and environmentally benign in terms of energy consumption) operation of a continuously operating plant. As well as the “intended” changes in load already mentioned (e.g. reduction in load as a consequence of a fall in demand), there are also changes in load that result inevitably as a consequence of a desired change in the product composition. This is the case, for instance, when achievement of a desired change in the product composition is to significantly increase the ratio of total aniline used to total formaldehyde used (also referred to in the literature as A/F ratio), i.e. significantly more aniline would have to be introduced into the reactor for the same amount of formaldehyde. Since the dimensions of the reactor are fixed, however, it is not possible to increase the amount of aniline arbitrarily. In order to achieve the desired A/F ratio, it may therefore be necessary to reduce the formaldehyde feed and hence the production capacity (the “load”). If the A/F ratio is to be significantly lowered and this is brought about by an increase in the formaldehyde feed—with constant aniline feed—this is inevitably associated with a change in load. In the case of this type of changes in load, there are particular operation-related challenges since two factors of significance for the operation of a production plant—namely firstly the change in load itself, which in continuous production is inevitably associated with a change in residence time, and secondly the desired change in the product composition which, even without a change in load, would entail changes in the reaction parameters—occur simultaneously. This is of course also true when a change in load, in the event of an intended alteration in the product composition, is not necessarily required but is actually desired for other reasons (for example because there is lesser or greater demand for a product of different composition by comparison with the product in the customary composition).
The product composition of a mixture of di- and polyamines of the diphenylmethane series is characterized by two factors in particular, namely:                1. by the proportion by mass of the diamine of the diphenylmethane series (MMDA) based on the total mass of di- and polyamines of the diphenylmethane series (MDA), ωMMDA hereinafter, (“bicyclic content”) and        2. by the isomer distribution within the MMDA content, which—since 2,2′-MDA is usually present only in minor proportions—is appropriately described by the proportion by mass of 2,4′-methylenediphenylenediamine (ω2,4′-MDA) based on the total mass of diamines of the diphenylmethane series (also referred to as methylenediphenylenediamines).        
The present invention is concerned with a change in the first factor. The proportion by mass of diamines of the diphenylmethane series in the MDA is typically determined by high-performance liquid chromatography (HPLC), which is preferably also employed in the context of the present invention.
It would be desirable to have available a process for preparing di- and polyamines of the diphenylmethane series in which it is possible by simple measures to configure a change in the bicyclic content in conjunction with a change in load such that operation of the process can be continued in an optimized manner (for instance with regard to yield, time taken, energy consumption and avoidance of process-related problems such as caking or blockages in apparatus) in economic, environmental and operational aspects.